The present invention relates to a polymer whose solubility in water is changed depending on a very slight difference in the concentrations of a salt and more specifically to a water-soluble polymer which is soluble in the usual tap water, but is insoluble in an aqueous solution which contains not less than 0.5% by weight of a neutral inorganic salt comprising a monovalent ion such as NaCl, KCl and NaBr.
It has been known that when an inorganic salt as an electrolyte is added in an increasing amount to an aqueous solution of a water-soluble polymer, the polymer is precipitated out due to the salting-out phenomenon. On the other hand, it has also been known that salts are included in body fluids such as the human urine in an amount of not less than 0.5% by weight (the term "% by weight" will hereinafter be referred to as simply "%"). Under such circumstances, there has been developed a polymer which is soluble in tap water, but insoluble in an aqueous solution containing salts in an amount of not less than 0.5% and the polymer has been used as a barrier film which is insoluble in the body fluids, but soluble in tap water and capable of being thrown into the flush toilet. However, anionic polymers such as sodium salts of polyacrylate and carboxymethyl cellulose are less sensitive to an increase in the concentration of an electrolyte in an aqueous solution. Such anionic polymers become insoluble in, for instance, an aqueous solution of common salt having a concentration of 4 to 5% or higher. Moreover, non-ionic polymers such as hydroxyethyl cellulose and polyvinyl alcohol (PVA) are also soluble in such an aqueous salt solution up to a more higher salt concentration and, for making these non-ionic polymers insoluble in an aqueous solution, the concentration of the salt, for instance, common salt must be increased to about 10% or higher. Therefore, these polymers as such cannot be used for forming the foregoing liquid-tight film. For this reason, many investigations for solving these problems have been made. For instance, Japanese Patent Unexamined Publication (hereinafter referred to as "J. P. KOKAI") No. Sho 50-52371 discloses an alkali cellulose ether sulfate as a polymeric material whose solubility varies depending on the concentration of a salt in an aqueous solution. However, the difference between solubilities of the sulfate in tap water and in a 0.5% aqueous common salt solution is not clearly described in this patent. Moreover, the alkali cellulose ether sulfate is prepared by subjecting a cellulose derivative such as ethyl cellulose to sulfation and, therefore, the resulting polymer is relatively expensive.
As a means for solving the foregoing problems, J. P. KOKAI No. Sho 63-139906 discloses a partially neutralized copolymer of acrylic acid or methacrylic acid with an alkyl (meth)acrylate and it has been reported that this polymer is soluble in tap water, but insoluble in an aqueous solution of common salt having a concentration of not less than 0.5%. Although this polymer makes it possible to solve the foregoing problem, it has high adhesion and cannot be used in the form of a film or a binder since films are very sticky and are hence liable to adhere to one another.